1. Field of the Invention
The present invention relates to barium titanates, strontium titanates, or mixed barium and strontium titanates, consisting essentially of solid, spherical, non-aggregated particles, and to a process for their preparation
More particularly, this invention relates to a new powder of the foregoing titanate consisting essentially of nearly perfect spherical particles having a very narrow particle size distribution (monodispersed powders), having a diameter ranging from 0.05 to 1 micron (micrometer), and exhibiting good dispersibility.
2. Description Of The Prior Art
Titanates have a broad range of uses, such as, for example, as ceramics for electronic components, such as, for instance, dielectric ceramics, piezoelectric ceramics, semiconductive ceramics, pyroelectric ceramics, etc.
A number of these applications are cited, for example, in "Fine Particle Perovskite Processing" by K. S. Mazdiyasni, Vol. 63, No. 4 (1984), page 591, Am. Ceram. Soc. Bull.
As is well known, in the preparation of sintered parts for general electronic uses, it is necessary that the titanate be present in the form of spherical particles, that the particle size not exceed 1 micron in diameter, and that the particles not be in the form of aggregates, so as to have good dispersibility in order to obtain uniform sintering during the preparation of ceramic bodies, with consequent uniformity of the density of the packed or pressed powder (green density).
One of the main problems in obtaining high-technology ceramics is securing a starting powder which can be used to obtain ceramic systems with high reproducibility and reliability.
It is well known that the degree of reproducibility is related to the ability to control the microstructures in the sintered bodies which, as is also known, depends on the characteristics of the starting material.
If the particles have essentially a perfectly spherical form and a narrow particle size distribution (monodispersed particles), the reproducibility characteristics are markedly improved.
In brief, the basic starting powder parameters which enable one to obtain high-technology ceramic systems are:
the particle diameter, which must not exceed 1 .mu.m (1 micron);
the particle size distribution, dw/dn, which should be as narrow as possible, dw/dn not exceeding 1.20, as defined hereinafter;
the particle shape, which should exhibit essentially perfect sphericity; and
the particle agglomeration, which should be essentially absent.
A powder having all the above characteristics permits a uniform packing of the particles so as to minimize voids between the particles, whereby the resulting density of the packed or pressed powder exhibits maximum uniformity.
Such uniform packing and density provide several advantages, including shorter sintering times, lower sintering temperatures, and a more uniform shrinkage as reflected by a uniform densification of the microstructure, with the final density of the system approaching the theoretical density.
As previously noted, all of the foregoing advantages are a function of the characteristics of the starting powder. This is well known and is described in detail in various scientific articles, including "Emergent Process Methods for High-Technology Ceramics", Materials Science Research, Vol. 17 (1984 Ed.) Robert F. Davis et al, Plenum Press, New York and London; R. L. Pober et al, "Dispersion and Packing of Narrow Size Distribution Ceramic Powders," page 193; "Ultrastructure Processing Ceramics, Glasses and Composites," Larry Hench and Donald R. Ulrich, Editors, John Wiley & Sons, (1984 Ed.), Chapter 2, page 6. However, titanate powders having a combination of all of the characteristics referred to above have not been obtained in the art.
The desired high-technology ceramic systems can only be obtained by syntheses methods which result in powders which exhibit the indicated combination of characteristics. The classification methods which can be utilized and are presently utilized to obtain powders consisting of particles having approximately the same diameter do not in fact enable one to obtain monodispersed particles. This has been ascertained for various powders (other than barium titanate, which is not presently available in the art in the form of monodispersed spherical particles) such as, for example, titanium dioxide. See FIG. 4 at page 200 of the R. L. Pober article previously cited.
Many processes are known for preparing barium titanate, for example:
reaction, in the solid phase, of a barium compound, for example, a nitrate, oxalate, oxide, or carbonate, with TiO.sub.2 ;
thermal decomposition of complex salts of barium and titanium such as oxalates or citrates, obtained by reaction in the liquid phase; and
aging of a titanium gel in the presence of barium hydroxide at such concentrations as to obtain a strongly basic pH, by operating at temperatures ranging from 60.degree. to 100.degree. C. or higher.
By these known processes it is not possible, as already noted, to obtain spherical, monodispersed particles. A titanate from a precursor consisting of a complex salt of the oxalate requires high decomposition temperatures. This causes a caking of the particles, with the consequent drawbacks mentioned above. Further, the particles generated from a precursor complex do not exhibit a spherical morphology.
By reacting the above indicated compounds in the solid phase there are obtained agglomerated products, because the reaction must be carried out at high temperatures. Moreover, the particles are not spherical in form and have sizes in excess of 1 micron. They therefore require grinding post-treatments, which always give rise to highly impure polydispersed products. Other liquid-phase preparation methods are known, such as, for example, the method described in U.S. Pat. No. 4,061,583, in which there is used, as a precursor of the titanate, a peroxide complex which is converted to the titanate by heating to a temperature equal to or in excess of 100.degree. C. This process leads to products with sizes below 1 micron, but without the spherical form.
In published European patent application E.P. No. 141,551 there is described a method of preparing various titanates, in particular of BaTiO.sub.3, from a gel as described above, resulting in BaTiO.sub.3 particles having a more defined morphology than products previously known in the art, but which are not spherical. This method, which utilizes a heterogeneous system in which one of the components is in the solid phase, makes it difficult to control the morphology, although it does permit the utilization of much lower temperatures than when carrying out reactions in the solid phase. Further, the resulting titanate is in the form of polydispersed particles. The lack of perfect sphericity as well as the wide distribution of particle diameters results in the drawbacks previously described.
We repeated Example 1 of patent application EP No. 141,551. The results by examination with a transmission electron microscope (TEM) are shown in FIG. 8 (36,700 magnification), and in FIG. 9 by examination with a scanning electron microscope (SEM) (30,000 magnification). The diameter of the particles is 0.11 .mu.m, dw/dn is equal to 1.36, and the total mean axial ratio is 0.836.
From the powders obtained according to the methods of this European patent application it is not possible to prepare very thin films with regular microstructures, and therefore it is also not possible to obtain ceramic bodies having a high uniformity. This is a serious limitation of these powders, since it does not permit their use, for example, to produce high capacity capacitors. Further, the polydispersity adversely affects all of the subsequent molding steps used in making ceramic bodies because the stability of the dispersions is not constant over a period of time due to the coagulation of the smaller particles with the larger particles (Ostwald ripening). This effect results in a further worsening of the packing with a consequent major decrease in densification.
In addition, in packed products obtained by sedimentation, using gravity or centrifugation, a polydispersed product, because of the different particle sedimentation velocities, results in non-uniform, stratified films. This is described in detail in the previously cited articles.
Further, the process employed in said European patent application requires one to operate in an entirely CO.sub.2 -free atmosphere. The presence of CO.sub.2 would cause partial formation of BaCO.sub.3 along with BaTiO.sub.3, resulting in composition nonhomogeneities and in considerable morphological irregularity of the titanate. The formation of BaCO.sub.3 is to be avoided because it leads to agglomeration of the BaTiO.sub.3 powder. Temperatures of about 700.degree. C. are required for its complete removal. Operating in an atmosphere free of CO.sub.2 requires a lengthy period of time in order to obtain the final BaTiO.sub.3 product. This is apparent from the examples of said European patent application.
Thus, there has been a need for a simple method capable of providing titanates having the foregoing characteristics, primarily in the form of fully spherical, uniform particles.